pH-driven Loading of Resveratrol Into Medium-chain Triglyceride Oil-in-water Emulsions: Impact of pH, Temperature, and Droplet Size on Properties and Stability

Abstract

Resveratrol is a bioactive polyphenol with well-documented antioxidant and health-promoting properties, but its poor water solubility and susceptibility to environmental degradation limit its use as a functional ingredient. This study explored how droplet size, pH, and storage temperature affected the partitioning and stability of resveratrol-loaded medium-chain triglyceride (MCT) oil-in-water emulsions. The resveratrol was loaded into the oil droplets using a pH-driven method, which is based on the substantial decrease in resveratrol solubility when the aqueous solution is changed from alkaline to neutral. Initially, oil droplets with different mean droplet diameters (D₃₂) were prepared by varying the homogenization method and conditions: small (0.62 μm), medium (1.29 μm), and large (6.79 μm). There was a slight increase in droplet size for the emulsions containing small droplets after incorporation of the resveratrol. Moreover, the surface charge of the small and medium droplets became more negative after loading the resveratrol, which suggests that some of this substance may have been located at the oil-water interface. The solubility of resveratrol in MCT oil, its recovery after emulsion preparation, and the oil/water partitioning were measured using high-performance liquid chromatography (HPLC). The maximum solubility of resveratrol in MCT oil was 2.03 mg/mL at 37 °C. The partitioning of resveratrol into the droplet-associated fraction (oil droplets + interfacial layers) was highest for the emulsions containing the smallest droplets (> 74%). In general, the recovery of the resveratrol in the emulsions was relatively high (> 93%), suggesting there was little loss of resveratrol during the loading process. All the emulsions remained stable throughout 21 days of storage at 4 °C, with little resveratrol loss and little change in droplet characteristics. However, storage at 37 °C caused some resveratrol degradation, especially in the emulsions containing the larger droplets. The resveratrol remained relatively stable when stored at pH values from 3 to 7 but exhibited appreciable degradation when stored at pH 8 and 9, highlighting the poor stability of this polyphenol under alkaline conditions. When exposed to UV light, resveratrol degradation followed second-order kinetics. The photostability of resveratrol was strongly affected by pH, with a much shorter half-life under alkaline conditions than at other pH values. Slightly greater resveratrol stability was observed in emulsions containing smaller oil droplets, but the effects were not statistically significant. Our findings show that resveratrol encapsulated in emulsions is more stable when stored under mildly acidic to neutral storage conditions, especially at refrigerated temperatures, with an additional, more modest benefit from reducing the oil droplet size. These results therefore provide practical guidance for the formulation of resveratrol-based functional foods, nutraceuticals, and cosmeceuticals.